The present invention relates to photolithography technology, and more particularly, to a phase shift mask with enhanced resolution and a method for fabricating the same.
With increases in an integration degree of a semiconductor device, a critical dimension of circuit patterns becomes finer and finer. In order to form these fine patterns, a phase shift mask capable of enhancing a resolution is used in an exposure process. A dose of exposure light incident to high density patterns in which a gap between the patterns is relatively narrow is relatively raised, and a defect that patterns with a desired size cannot be formed can be caused as a dose more than a proper level is undesirably incident. This can be caused as the dose incident on respective patterns affects adjacent patterns due to the increased pattern density and thus the dose incident on the wafer is actually increased.
In order to restrict this increase in the dose, conventional solutions have included reducing the dose of an exposure light source itself used in the exposure process and using a high sensitive photoresist exposable by the exposure light source with the reduced dose. However, light incident on the wafer through a phase shift layer of the phase shift mask also has the dose more than a predetermined level; a peripheral portion of the light is extinguished by destructive interference with other light passed through an adjacent light transmitting portion but the dose of the light passed through the middle portion of the phase shift layer is transferred onto the wafer as it is. Therefore, when using the photoresist with higher sensitivity, unnecessary dose is incident on the photoresist to expose the photoresist. Consequently, this functions as a restriction factor obstructing the formation of finer patterns.
FIG. 1 is a sectional view illustrating a phase shift mask and FIG. 2 is a view showing a dose and a phase of light transmitted through the phase shift mask of FIG. 1.
Referring to FIG. 1, the phase shift mask is provided with a phase shift layer pattern 13 such as a molybdenum (Mo)-silicon (Si) alloy layer on a transparent quartz substrate 10. The phase shift layer pattern 13 has a shape for providing the shape of circuit patterns to be transferred on the wafer and an exposed region of the substrate 10 is set up as a light transmitting part 11.
Referring to FIG. 2, in distributions in phase and dose of the exposure light incident on the phase shift mask and transferred onto the wafer during the exposure process, it is effective that a first dose 21, an intensity of a first light transmitted through the light transmitting part 11 is more than a critical dose and a second dose, an intensity of a second light transmitted through the phase shift layer pattern 13 is lower than the critical dose. At this time, a phase 31 of the first light is an opposite phase (phase difference of 180°) to a phase 33 of the second light and induces that, in a destructive interference region, i.e. a boundary region, a destructive interference occurs therebetween and thus the light dose becomes substantially zero. Accordingly, a contrast in the boundary region of the phase shift pattern 13 is increased.
However, the portion of the second light transmitted through the middle portion of the phase shift layer pattern 13 is not destructed but transferred onto the wafer. The second dose 23 or an intensity of the second light transmitted through the phase shift layer pattern 13 should be controlled to have a dose less than the critical dose which is the limitation not exposing the photoresist on the wafer, but an over dose phenomenon, in that the second light dose 23 exceeds the critical dose by the reduction of the critical dose, can be caused when using a photoresist with higher sensitivity required to form finer patterns. In this case, there can occur an exposed pattern defect in that the region of the photoresist corresponding to the phase shift layer pattern 13 is also exposed and patterned.
Accordingly, it may be contemplated to reduce the dose 23 of the second light transmitted through the phase shift layer pattern 13 by increasing a thickness of the phase shift layer pattern 13. However, since a quartz substrate is used in the fabrication of the phase shift mask and a variation in the thickness of the phase shift layer pattern 13 causes a phase shift of the second light transmitted through the phase shift layer pattern 13, the variation in the thickness of the phase shift layer pattern 13 can lead to lowering in performance of the phase shift mask. Therefore, it is required to develop an enhanced phase shift mask capable of reducing the light dose 23 of the second light transmitted through the phase shift layer pattern 13 with variation in the thickness or material of the phase shift layer being restricted.